Diversity and distributions of the submarine-cave Neritiliidae in the Indo-Pacific (Gastropoda: Neritimorpha)

Sediment samples from approximately 100 submarine caves on tropical islands in the Indian, Pacific and Atlantic oceans were examined to elucidate the global diversity and distribution of obligate submarine-cave snails of the family Neritiliidae. Shells accumulated from the Indo-West Pacific samples comprise five genera and nine species of extant neritiliids, whereas there were none from the Atlantic. Four new genera and four new species are herewith described: Laddia lamellata, Micronerita pulchella, Teinostomops singularis and Siaesella fragilis; previously known species include Laddia traceyi comb. n., Pisulina adamsiana, Pisulina biplicata, Pisulina maxima and Pisulina tenuis. Of these nine species, seven have wide, largely overlapping distributions; species richness is highest in and around the Indonesian and Philippine region, as in countless cases of shallow-water fishes, corals, echinoderms, bivalves and other gastropods. Examination of protoconch morphology revealed five species with a fairly long, planktotrophic larval period and four species with non-planktotrophic early development. No clear relationship was found between distribution range and dispersal capability deduced from the developmental mode, whereas the non-planktotrophs had higher levels of geographic differentiation in shell morphology. Fossil assemblages from cryptic environments suggest a Tertiary origin of the submarine-cave Neritiliidae. The oldest extant genus, Laddia, appeared in the Miocene, while two other Tertiary genera, Bourdieria and Pisulinella, have become extinct. The submarine-cave Neritiliidae thus do not seem to have remained in the same cave systems or the same local regions for millions of years, but seem to be relatively young, active colonizers of both continental and oceanic islands, having repeatedly expanded their distributions over the Indo-West Pacific. Despite the fact that they are undoubtedly restricted to caves and similar cryptic habitats, transoceanic dispersal appears to have played as important a role in forming present distributions as did tectonic events, in species with or without a planktotrophic larval period.